It is known that high strength nickel, cobalt or iron based superalloys, for example nickel based superalloys with other elements such as aluminum and titanium, have their high strength characteristics because of the precipitation hardening effect of the high degree of gamma prime phase in the material. It is also known that those superalloys are very difficult to weld successfully.
In document US 2005/0194363 A1 a multi-laser beam welding of high strength superalloys is described. The method uses an array of two or more lasers to perform the steps of heating, welding (wherein a superalloy powder is added as a filler material) and post-weld heat treatment in nearly simultaneous operation. The secondary laser spot heats the area that has just been welded, thus slows the rate of cooling encountered at the weld spot which helps to reduce or even to eliminates the hot cracking, but because of using a filler material during the welding it is expected that the strength properties of the weld are not sufficient.
Therefore, high strength welds are often done by laser welding, electron beam welding, or similar processes that can weld gamma prime strengthened superalloys without a filler material. Welding without a filler material has the advantage that the weld has a similar strength like the base material, which is joined. But it has the disadvantage that due to the rapid cooling inherent to the welding process few or no gamma prime precipitates are present in the weld.
This is the reason, why post-weld heat treatments are often necessary to adjust the microstructure of the parts. During that post-weld heat treatment the gamma prime phase also precipitates in the weld region. These precipitations cause crystallographic changes in the alloy resulting in smaller lattice parameter after heat treatment. The volume change associated with this precipitation can lead to cracking in the weld region which is called strain age cracking or post-weld heat treatment cracking.
Document U.S. Pat. No. 7,854,064 B2 discloses a method for repairing a turbine engine component. The method comprises the steps of providing a turbine component made of a cast or wrought Nickel base superalloy, including for example Waspaloy, IN-738, IN-792, IN-939, removing any defects from the component wherein the removing step comprises a solutioning heat treatment step with a rapid heating rate between 29 and 40° F./minute (16-23° C./min) as the component passes through a temperature in a range from 1100 to 1600° F. (593-871° C.), wherein the component and the replacement parts are no longer in that mentioned temperature range than 17 minutes. After the component has reached the desired solutioning temperature, it is held at temperature for about 3 to 5 hours, then cooled, and then welded by electron beam welding, plasma arc welding or gas tungsten arc welding (GTAW) to effect the repair. A slow post-solution treatment cooling rate with a cooling rate of 0.5 to 10° F./minute (0.3-5.6° C./min), preferable 0.5 to 1.0° F./minute (0.3-0.6° C./min) is used from solutioning temperature to below 1250° F. (677° C.) to substantially prevent weld cracks and enhancing weldability. The slow cooling rate shall allow a significant time for γ′ precipitation to occur during cooling and to grow significantly. This reduces their hardening capability and increases high temperature ductility.
In addition, it is disclosed in document U.S. Pat. No. 7,854,064 B2 that the welded turbine component could be subjected to a post-weld heat treatment. The same heating and cooling rates for the post-weld heat treatment are used as for the above-mentioned pre-weld heat solutioning treatment, that means that a rapid heating rate in the range from 29° F./minute to 40° F./minute (16-23° C./min) is used as said welded turbine engine component is in a temperature range of from 1100 to 1600° F. (593-871° C.), and the cooling of said welded turbine engine component from the maximum solution heat treatment temperature to a temperature below 1250° F. (677° C.) is done very slowly at a cooling rate from 0.5 to 10° F./minute, preferable from 0.5 to 1° F./minute (0.3-5.6, preferably 0.3-0.6° C./min). This modified post-weld heat treatment is applied to eliminate the microstructural features generated by the pre-weld solution treatment's slow cooling rate and restore specification mechanical properties.
Although the method disclosed in U.S. Pat. No. 7,854,064 B2 has the advantage that turbine components made of Nickel based superalloys could be repaired e.g. welded virtually without the presence of microcracks it has the disadvantage of being time and cost consuming with respect to the described multiple steps of pre-weld and post-weld heat treatment.
Document US2012/0205014 A1 describes an inertia friction weld of superalloys with enhanced post-weld heat treatment. Friction welding avoids solidification cracking, but because of the cold working during friction welding (plastically deformation of at least one of the deposit material and the superalloy substrate) residual stresses are induced. Therefore, a heat treatment is proposed comprising a post-weld intermediate stress-relief (ISR) treatment, followed by a solutioning treatment, followed by a precipitation hardening heat treatment. The ramp time to 870° C. is about 102 minutes for the ISR, that means a heating rate of about 8 to 9° C./min, which is relative low.